This invention deals generally with building construction and more specifically with the construction of concrete prefabricated walls.
The traditional methods of constructing building basements are well established. For commercial structures and for high volume residential developments with identical dimensions for each building, poured concrete is used. This involves the construction of forms, either wood or metal, in the exact shape of the vertical basement walls, and then pouring concrete into the forms. After the concrete hardens, the forms are removed and construction continues on the rest of the building.
The cost of forms limits this method to structures where the height requires the strength of reinforced concrete or where the reuse of forms for many identical structures in the same general area permits sharing of the costs of form construction by many buildings.
The more common basement construction technique for individual residential houses is the straight forward construction of the vertical walls by laying many courses of cinder block, one on top of the other. This method is the traditional one used for isolated building sites and small developments, and it is both time consuming and labor intensive. It requires each cinder block to be individually placed and surrounded with mortar. One need only watch a traditional house being built to realize that the cinder block basement may take over a week to construct on a typical site, while the framing and exterior walls go up in just a day or so. Above ground walls of wood and sheathing have been prefabricated for over forty years, but prefabrication of concrete walls has only started very recently.
U.S. Pat. No. 4,605,529, issued Aug. 12, 1986 to Melvin M. Zimmerman, the present inventor, discloses a prefabricated concrete wall structure and the method and assembly jig for the wall's construction. The method disclosed permits construction of a wall which is no longer linked to the amount of manpower available at the construction site, because the labor at the construction site involves only installation of the previously manufactured wall. Moreover, the cost of the wall is relatively unrelated to the size of the wall.
However, the cost of such a prefabricated wall is greatly influenced by the ease of its off-site manufacture, and to facilitate construction the wall is constructed within a horizontal assembly jig, so that conventional concrete delivery trucks can be used as a source of material.
The assembly jig of the prior art is essentially a set of channel members oriented in a horizontal plane. The channels are arranged in parallel, about eight feet apart, and the channels include precut notches on their innermost walls. These notches are used to support previously manufactured concrete studs which are set in the horizontal plane perpendicular to the parallel channels. A typical spacing for the notches is two feet center to center.
The channel members and frame sides joining the ends of the channel members are constructed so that the peripheral edges of the grid formed by the studs and the channels, that is, the edges forming an outside rectangle, are higher than all the other members by approximately four inches to form a frame around the entire structure. The parallel channel members which form the support for the concrete studs include cavities of considerable volume which are eventually filled with concrete to encase the ends of the concrete studs which are set into the notches in the channels with the ends of the studs extending into the cavities.
Before concrete is poured into the assembly jig, sheet insulation is laid over the concrete studs and impaled upon fasteners cast into and protruding from the concrete studs, and wire reinforcing mesh is laid atop the sheet insulation, but the sheet insulation is sized so that it does not cover the cavities of the channel members.
The wall is then completed by pouring concrete into the jig so that it covers the insulation, the fasteners protruding through the insulation, and the wire mesh, and fills the cavities in the channel members. The concrete is poured to the height of the top of the outer frame members, and once hardened, not only forms an integral exterior surface, but also bonds together the studs, the insulation, and the top and bottom support beams which are formed in the channel members.
The prior art describes the final step of manufacture of the wall as lifting the hardened concrete wall from the assembly jig by jacking one edge of the wall out of the assembly jig, and then attaching lifting aids, such as eyebolts, to holes cast in the upper or lower concrete beams.
However, the task of removing the finished wall from the assembly jig disclosed in the prior art is much more difficult than one might suppose. There is a significant tendency for the wall to adhere to the jig and lock up within it. For example, concrete may leak through the edges of the notches cut in the channel members to support the studs, and such concrete, once hardened, prevents removal of the wall from the assembly jig.
Moreover, even slight irregularities in the sidewalls of the channel members also tend to lock the wall into the assembly jig. For instance, if a dent exists in the sidewall of a channel member, that dent will, depending upon the direction in which it protrudes, either be filled with or surrounded by hardened concrete. Under such circumstances, it is impossible to pull out the concrete beam formed within the channel member without chipping, or at least scoring, the concrete beam. Furthermore, even if the damage to the prefabricated concrete wall can be tolerated, the rigid channel members of the assembly jig of the prior art require much more force, more powerful equipment, and more disassembly time than is desirable to remove the finished wall from the assembly jig.